Polymer-based burn-out material for the lost-wax technique

ABSTRACT

Modelling material which includes (a) at least one radically polymerizable monomer, (b) at least one initiator for the radical polymerization and (c) at least one inert component. The inert component (c) is soluble in the polymer formed by polymerization of the monomer (a), wherein the solubility of component (c) decreases as the temperature increases, with the result that a phase separation takes place above a particular temperature. The material is suitable in particular for the production of models of dental restorations for investment casting processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. Ser. No. 15/832,841, filed Dec. 6, 2017, which claims priority toGerman Patent Application No. 102016225208.9 filed on Dec. 15, 2016, allthe disclosures of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to materials which are suitable inparticular for the production of models for investment casting processesby additive processes and in particular by stereolithographic processes.Investment casting processes are used for the production of shaped partswhich require a high degree of accuracy, for example for the productionof decorative objects or dental restorations.

BACKGROUND OF THE INVENTION

For the production of dental restorations according to the conventionallost-wax technique, a wax model (wax-up) is shaped, invested in aninvestment material and heated up in a furnace after this has set. Thewax melts and flows out of the mould. The mould is then heated furtherwhereby wax residues burn. In this way, a negative of the wax model isobtained into which an alloy or a ceramic material is then poured. Thecast object is then again a positive which is exactly the same as thewax model.

The production of the wax model can be carried out in different ways. Inthe case of manual modelling, a model of the tooth restoration is builtup by the dental technician in layers on a plaster model using a specialdental wax. The waxes soften on warming and, after melting, flow out ofthe investment material, normally without causing cracks. This processis still the gold standard with regard to the quality of cast andpressed objects.

In newer processes, the wax model is milled out of wax discs or blocksusing CAD/CAM processes. The waxes used here require a compromisebetween good milling qualities and ability to burn out. The productswhich are commercially available today for the most part meet bothrequirements sufficiently well. These waxes also soften on warming and,after melting, flow out of the investment material without causingcracks.

Wax models are relatively delicate with the result that, increasingly,alternative materials are used for the production of models. In the caseof CAD/CAM processes, plastic discs and blocks as well as plastic-waxhybrid discs are used, for example. In the case of manual modelling,curable resins, which can be cured cold by mixing two components,thermally or preferably by light, are used as alternatives to waxes.These materials contain relatively large quantities of non-meltableduromers.

Recently, so-called additive processes are increasingly being used forthe production of models. By this is meant manufacturing processes inwhich three-dimensional models or components are produced fromcomputer-aided design data (CAD data) (A. Gebhardt, Vision of RapidPrototyping, DGK Report 83 (2006) 7-12). These are processes such ase.g. stereolithography (SL), selective laser sintering (SLS), 3Dprinting, fused deposition modelling (FDM), ink-jet printing (IJP), 3Dplotting, multi-jet modelling (MJM), solid freeform fabrication (SFF),laminated object manufacturing (LOM), laser powder forming (LPF), withwhich models, components or shaped parts can be producedcost-effectively even on a small scale (A. Gebhardt, GenerativeFertigungsverfahren [Additive Manufacturing Processes], 3rd ed., CarlHanser Verlag, Munich 2007, 77 et seq.). In the case ofstereolithography, a shaped part is built up in layers from a liquid andcurable monomer resin on the basis of CAD data (A. Beil, Fertigung vonMikro-Bauteilen mittels Stereolithographie [Manufacture ofmicro-components using stereolithography], Dusseldorf 2002, VDI-Verlag 3et seq.). Additive processes are often covered by the term “RapidPrototyping” (RP).

A disadvantage of the materials used as alternatives to waxes is theirmelting behaviour. Pure waxes first of all soften on warming, then meltand subsequently flow out of the mould. During the thermally causedexpansion, they do not therefore exert large forces on the investmentmaterial. The alternative materials do not exhibit this meltingbehaviour. They expand thermally during the setting/curing of theinvestment material and the temperature rise associated therewith aswell as during the subsequent heating, however do not flow out of themould but are primarily removed therefrom by thermal decomposition.During the expansion, they therefore exert pressure on the investmentmaterial, which leads to stresses and, in particular in the case oflarge-volume dental restorations, to cracks in the negative mould, whichresult in pressing and casting defects and, in the case of largercracks, also in faulty pressings or faulty castings.

It is known to add wax particles to curable materials. These areintended, during setting of the investment material or during burn outin the furnace, to melt and thus to create space for the expandingmaterial. However, often the molten wax cannot escape or can only escapewith difficulty because it is enclosed by the polymerized material andbecause the diffusion coefficient of the molten wax molecules throughthe polymer matrix is only very small. In the least favourable case, thelarge increase in volume of the waxes on melting actually increases thethermal expansion of the model instead of reducing it.

SUMMARY OF THE INVENTION

The object of the invention is to provide materials for modelling dentalprostheses which do not exhibit the disadvantages named above. They areintended to enable the production of models, in particular bystereolithographic processes, which can be removed without the formationof cracks after the investment material has set, with the result thatfault-free moulds and restorations are obtained.

DETAILED DESCRIPTION

This object is achieved according to the invention by radicallypolymerizable compositions which contain (a) at least one radicallypolymerizable monomer, (b) at least one initiator for the radicalpolymerization and (c) at least one inert component.

The compositions according to the invention are characterized in thatthe inert component (c) is soluble in the polymer formed bypolymerization of the monomer (a), wherein the solubility of component(c) decreases as the temperature increases, with the result that a phaseseparation takes place above a particular temperature. This temperatureis referred to here as critical temperature or Lower Critical SolutionTemperature (LCST). Component (c) is an organic compound.

Under standard conditions (20° C., 1013 mbar), component (c) is liquidor solid and has a pour point which is below the LCST. It preferably hasa pour point of above −150° C., particularly preferably of above −100°C. and quite particularly preferably of above −50° C.

According to DIN 51597, for a liquid product, the pour point refers tothe temperature at which, on cooling, it only just flows. The settingpoint according to DIN 51583 refers to the temperature at which thepreviously liquid product solidifies. The pour point is determined inaccordance with DIN 51597/ISO 3016.

Component (c) preferably has a boiling point of above 80° C., preferablyabove 120° C., particularly preferably above 150° C. Components which donot boil under standard pressure without decomposition preferably have athermal decomposition temperature of above 120° C., preferably above135° C., particularly preferably above 150° C. The relatively highboiling and decomposition temperature ensures that component (c) is alsosolid or liquid under conditions of use and, for example, does notevaporate when the investment material is printed or sets. Compoundswith a maximum boiling temperature of 350° C. and/or a maximumdecomposition temperature of 500° C. are preferred.

Component (c) is chemically inert, this means that it does not react ordoes not markedly react with the other components of the material understorage conditions and conditions of use. In particular, component (c)does not contain any radically polymerizable groups and therefore doesnot copolymerize with the monomer (a). It is homogeneously miscible withcomponent (a), and mixtures of (a) and (c) are stable at the processingtemperature, i.e. no phase separation takes place, in particularcomponent (c) does not precipitate out or crystallize out duringprocessing. The processing of the materials according to the invention,i.e. the production of models, for example by stereolithographicprinting, preferably takes place at a temperature of 15 to 35° C.,preferably 15 to 30° C. Materials are preferred that can be processedand stored at room temperature (20° C.) so that the materials do notadditionally need to be warmed for processing or do not need to becooled for storage.

Compounds which burn without residue in particular come intoconsideration as component (c). By this is meant substances whichpreferably form less than 0.1 wt.-% ash on burning. Compounds arepreferred which burn on heating in an oxidizing atmosphere to atemperature of approx. 850° C.

The compositions according to the invention contain, as component (a),at least one radically polymerizable monomer and, as component (b), atleast one initiator for the radical polymerization. They are preferablyin the form of a homogeneous mixture of components (a), (b), (c) andoptional constituents that may be present. They can be cured thermally,via a two-component initiator system or photochemically, depending onthe initiator used. Compositions which contain a photoinitiator arepreferred.

According to the invention, materials for the production of models bymanual modelling are preferred, by additive manufacturing processes areparticularly preferred and by stereolithography are quite particularlypreferred. By a model is meant here in particular the desired positivemould for investment casting processes. Although, unlike in theconventional lost-wax technique, the model is primarily removed from themould by burning out after the investment material has set, the processhere is referred to as investment casting. The material according to theinvention is suitable in particular for the production of models formanufacturing dental restorations.

After curing below the critical temperature, component (c) is integratedstably in the material and thus enables problem-free further processingof the material. Above the critical temperature, a phase separationtakes place and at least some of component (c) bleeds out of the curedmaterial. This temperature-induced bleeding-out reduces the thermalexpansion of the material with the result that, even in the case oflarge-volume models, no cracks form in the investment material andpressed and cast objects without pressing and casting defects can beobtained.

FIG. 5 shows the thermal expansion behaviour of a conventional material.Conventional materials expand thermally on warming and the volumeincreases continuously. Above a particular temperature, the materialsstart to decompose. The decomposition manifests itself in a reduction involume. The decomposition temperature is material-dependent and, formost organic materials, is above 300° C.

On warming, most materials according to the invention first also exhibita slight expansion, but this turns out to be considerably smaller thanin the case of conventional materials (FIGS. 1-4).

The expansion curves already fall again significantly before thedecomposition temperature is reached. This reduction in volume is causedby the bleeding-out of component (c). The bleeding-out largely balancesout the thermal expansion of the material. Although a slight rise in thecurves is sometimes observed again at higher temperatures, this turnsout to be comparatively small. Overall, the materials according to theinvention expand over the entire temperature range up to decompositionconsiderably less than known materials.

The expansion curves pass through a maximum. Here, this maximum of thethermal expansion is referred to as maximum linear thermal expansion.

The modelling materials according to the invention are characterized bya relatively low maximum linear thermal expansion. This is preferablybelow 1.5%, particularly preferably below 1%, quite particularlypreferably below 0.7%. The linear thermal expansion is measured in thetemperature range from 30° C. to 800° C., wherein most materials alreadystart to decompose before 800° C. is reached. The decompositionmanifests itself in shrinkage of the measurement bodies.

It is particularly advantageous for the maximum expansion to be reachedat a temperature of below 150° C., preferably below 120° C.,particularly preferably below 100° C. and quite particularly preferablybelow 90° C. In contrast, commercially available burn-out SL resinstypically have a maximum thermal expansion of 2% or more which is, inaddition, only reached at relatively high temperatures.

The thermal expansion is preferably determined on cylinders with a 6 mmdiameter and a height of 6 mm in the temperature range of from 30° C. to800° C. at a heating rate of 5 K/min in an air atmosphere. The measuringprobe is applied to the sample with a contact force of 0.1 N. The linearexpansion of the cylinders during heating up and thermal decompositionis measured. The measurement can take place e.g. using a Q400thermomechanical analyzer (TMA) from TA Instruments with amacro-expansion probe. The length of the cylinder at 30° C. is taken asthe original length, i.e. 0% expansion.

It is not normally necessary to use a particular temperature profile forbleeding out component (c) and no additional process step is required.After the investment material has set, the invested model can be placeddirectly in a furnace which has been preheated to the temperaturerequired for the melting out and/or burn-out of the model. The meltingout and/or burn-out typically takes place at a temperature of approx.850° C. The temperature cycle recommended by the investment materialproducer is preferably used. Often, the model has already completelyburnt or flowed out after an hour. The maximum residence time in thefurnace can vary depending on the investment material and the producer.In most cases, it is sufficient to leave the mould in the furnace at themaximum temperature for 1 to 8 hours. The heating-up process can takeseveral hours (1-12 h), and this period is sufficient to bring aboutbleeding-out of component (c) to the necessary extent.

The effect of the bleeding-out is presumably to be attributed to thefact that the solubility of component (c) in the polymer matrixdecreases as the temperature increases and above the Lower CriticalSolution Temperature (LCST) a noticeable phase separation takes place.Since component (c) still has a certain solubility even at highertemperatures in the polymer matrix formed by the radical polymerizationof component (a), however, the no-longer dissolved proportion candiffuse relatively rapidly through the matrix and thus also escape fromdeeper layers of the model.

During heating of the polymeric matrix, the molecules of component (c)located between the polymer chains can thus migrate at least partiallyout of the matrix before the polymerized proportions of the mixture aredepolymerized and pyrolyzed. The risk of defects such as cracks or eventhe destruction of the mould during burn-out is thereby at least greatlyreduced and for the most part eliminated completely. Through thedischarge of component (c), part of the organic mass is already removedduring the heating up at comparatively low temperature and the furtherheating procedure until all of the organic proportions have beencompletely removed can be carried out speedily. In this way, thematerials according to the invention produce crack-free moulds with ahigh degree of accuracy and precision.

The materials according to the invention preferably contain, asradically polymerizable monomer (a), at least one (meth)acrylate and/or(meth)acrylamide, preferably one or more mono- or multifunctional(meth)acrylates or a mixture thereof. Materials which contain at leastone multifunctional (meth)acrylate or a mixture of mono- andmultifunctional (meth)acrylates as radically polymerizable monomer areparticularly preferred. By monofunctional (meth)acrylates is meantcompounds with one, by polyfunctional (meth)acrylates is meant compoundswith two or more, preferably 2 to 6, radically polymerizable groups. Inall cases, methacrylates are preferred to acrylates.

Particularly suitable mono- or multifunctional (meth)acrylates aremethyl, ethyl, 2-hydroxyethyl, butyl, benzyl, tetrahydrofurfuryl orisobornyl (meth)acrylate, p-cumyl-phenoxyethylene glycol methacrylate(CMP-1E), bisphenol A di(meth)acrylate, bis-G(M)A (an addition productof (meth)acrylic acid and bisphenol A diglycidyl ether), ethoxylated orpropoxylated bisphenol A di(meth)acrylate, such as e.g. bisphenol Adi(meth)acrylate with 3 (SR-348c=methacrylate; SR-349=acrylate,Sartomer) or 2 ethoxy groups (SR-348L=methacrylate, Sartomer),2,2-bis[4-(2-(meth)acryloxypropoxy)phenyl]propane, UD(M)A (an additionproduct of 2-hydroxyethyl (meth)acrylate and 2,2,4- or2,4,4-trimethylhexamethylene-1,6-diisocyanate), di-, tri-, tetra-,penta-, hexa- or heptaethylene glycol di(meth)acrylate, di-, tri-,tetra-, penta-, hexa- or heptapropylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, ethoxylated or propoxylatedtrimethylolpropane tri(meth)acrylate, e.g. 3 times propoxylatedtrimethylolpropane triacrylate (Sartomer SR-492) and tripropylene glycoldiacrylate, pentaerythritol tetra(meth)acrylate, as well as glycerol di-and tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,10-decanedioldi(meth)acrylate (D₃MA), 1,12-dodecanediol di(meth)acrylate oroligomeric polyether, polyester, epoxy or urethane (meth)acrylates andtricyclodecane dimethanol di(meth)acrylate.

Particularly preferred are mono- and in particular di- and trifunctionalacrylates and methacrylates with a molecular weight of <1000 g/mol, suchas e.g. 2-phenoxyethyl (meth)acrylate, aliphatic urethane diacrylates,phthalic acid-HEA ester (Photomer 4173), pyromellitic acid-di-HEA ester(HEA=2-hydroxyethyl acrylate), bis-G(M)A (an addition product of(meth)acrylic acid and bisphenol A diglycidyl ether),2,2-bis[4-(2-(meth)acryloxypropoxy)phenyl]propane, UD(M)A, triethyleneglycol di(meth)acrylate (TEGD(M)A) and 2-phenoxyethyl (meth)acrylate(acrylate SR339C Sartomer/Arkema). These monomers are characterized by ahigh reactivity, a high double bond conversion, good mechanicalproperties, a low polymerization shrinkage and a relatively lowviscosity. Quite particularly preferred are those materials whichcontain, as component (a), UDMA, TEGDMA, bis-GMA (addition product ofmethacrylic acid and bisphenol A diglycidyl ether) or 2-phenoxyethylacrylate (SR339C, Sartomer/Arkema), a mixture of UDMA, TEGDMA and2-phenoxyethyl acrylate or preferably UDMA, TEGDMA and bis-GMA and inparticular a mixture of UDMA and TEGDMA.

The properties of the materials before and after the curing can beinfluenced by a targeted combination of monomers. Mixtures ofmonofunctional and difunctional monomers are characterized by arelatively low viscosity and reactivity of the resin mixture, whereinviscosity and reactivity decrease with the content of monofunctionalmonomers. A monofunctional monomer content ensures a lower rigidity ofthe models obtained by curing the materials. Mixtures of difunctionaland trifunctional monomers have a higher reactivity, wherein thereactivity increases with the content of trifunctional monomers. Thetrifunctional monomer content causes a higher brittleness. Reactivityand viscosity of the resin mixture and also the polymerization shrinkageare furthermore determined by the molar mass of the monomers, whereinthe polymerization shrinkage decreases with increasing molar mass, whilethe viscosity increases.

Preferred photoinitiators (b) for initiating the radicalphotopolymerization are benzophenone, benzoin and derivatives thereof orα-diketones or derivatives thereof, such as 9,10-phenanthrenequinone,1-phenyl-propane-1,2-dione, diacetyl or 4,4′-dichlorobenzil.Camphorquinone (CQ) and 2,2-dimethoxy-2-phenyl-acetophenone areparticularly preferably used, and α-diketones in combination with aminesas reducing agent, such as e.g. 4-(dimethylamino)benzoic acid ester(EDMAB), N,N-dimethylaminoethyl methacrylate, N,N-dimethyl-sym.-xylidineor triethanolamine, are quite particularly preferably used. Furtherpreferred are diethylthioxanthene (DETX, CAS 82799-44-8) andisopropylthioxanthone (ITX, CAS 75081-21-9), both in each casepreferably in combination with ethyl 4-(dimethylamino)benzoate (EMBO,CAS No. 10287-53-3). Likewise preferred are[1-(4-phenylsulfanylbenzoyl)heptylidenamino] benzoate (Irgacure OXE 01)and [1[9-ethyl-6-(2-methylbenzoyl)carbazol-3-yl]ethylidenamino] acetate(Irgacure OXE 02).

Particularly preferred photoinitiators are furthermore Norrish type Iphotoinitiators, above all monoacyl- or bisacylphosphine oxides, and inparticular monoacyltrialkyl- or diacyldialkylgermanium compounds, suchas e.g. benzoyltrimethylgermanium, dibenzoyldiethylgermanium orbis(4-methoxybenzoyl)diethylgermanium (MBDEGe). Advantageously, mixturesof the different photoinitiators can also be used, such as e.g.bis(4-methoxybenzoyl)diethylgermanium in combination with camphorquinoneand 4-dimethylaminobenzoic acid ethyl ester.

Quite particularly preferred are camphorquinone (CAS No. 10373-78-1) incombination with ethyl 4-(dimethylamino)benzoate (EMBO, CAS No.10287-53-3) as well as phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide(Irgacure 819, CAS 162881-26-7),diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide (TPO, CAS No.75980-60-8), 2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone(Irgacure 369, CAS No. 119313-12-1), 1-butanone,2-(dimethylamino)-2-(4-methylphenyl)methyl-1-4-(4-morpholinyl)-phenyl(Irgacure 379, CAS No. 119344-86-4) and quite particularlybis(4-methoxybenzoyl)diethylgermanium (MBDEGe; Ivocerin).

Phthalates, polyethylene glycols (PEG), polypropylene glycols (PPG),PEG-PPG copolymers, glycerol derivatives, i.e. in particular ethoxylatedand/or propoxylated glycerol, ethylenediamine tetrakispropoxylates,ethylenediamine tetrakisethoxylates and mixtures thereof are suitable inparticular as component (c). In all cases, the propoxylated compoundsare preferred to the ethoxylated compounds because ethoxylates tend, athigher molecular weights, to become solid and then require higherprocessing temperatures.

Component (c) can be solid at room temperature as long as it does notprecipitate out of the monomer used or out of the monomer mixture at theprocessing temperature. Substances are preferred which are pasty and inparticular liquid at room temperature. The consistency of component (c)can be adjusted, for example, by the proportion of PPG groups, e.g. bymixing propoxylated and ethoxylated compounds or by increasing theproportion of PPG groups within a molecule, e.g. in the case of PEG-PPGcopolymers. The proportion of PPG groups is preferably so high that thecomponent is liquid or pasty, particularly preferably so high that thepour point is lower than the processing temperature.

Component (c) preferably has a molecular weight of 1000 to 10,000 g/mol,particularly preferably 1000 to 5000 g/mol. Polyethylene glycols (PEG)preferably have a molecular weight of 1000 to 5000 g/mol, particularlypreferably 1000 to 3000 g/mol, polypropylene glycols (PPG) of 1000 to4000 g/mol, particularly preferably 1500 to 3000 g/mol and in particular2000 g/mol, PEG-PPG copolymers of 1000 to 10,000 g/mol, preferably 1500to 5000 g/mol and in particular 2000 to 4000 g/mol.

The ethylenediamine tetrakispropoxylates and -ethoxylates preferablyhave a molecular weight of 1000 to 10,000 g/mol, in particular of 1000to 5000 g/mol, wherein ethylenediamine tetrakispropoxylates and mixturesof ethylenediamine tetrakisethoxylates and -propoxylates are preferredto the ethylenediamine tetrakisethoxylates.

Unless otherwise stated, here the molar mass of polymers in all cases isthe molar mass Mu determined by viscometry (viscosity average).

Particularly preferred as component (c) are polypropylene glycols (PPG),in particular PPG with a molecular weight of 1000 to 4000 g/mol,preferably 1500 to 3000 g/mol, co-PEG-PPG with a molecular weight of1000 to 10,000 g/mol, preferably 1500 to 5000 g/mol and quiteparticularly polypropylene glycol with a molecular weight of approx.2000 g/mol.

Mixtures of the named substances can also be used advantageously. Inaddition to the named components, these mixtures can also containcompounds with a molecular weight of less than 1000 g/mol in a lesserquantity.

Terminal —OH groups of the named compounds can be esterified oretherified, for example with methyl, ethyl, propyl or iso-propyl groupsor formate, acetate, propionate or iso-propionate groups.

According to the invention, the type and quantities of components (a)and (c) are matched to each other such that the desired solubilitybehaviour is achieved. The solubility behaviour is influenced, amongother things, by the molecular weight of component (c). Theincompatibility with the polymerized matrix increases with increasingmolecular weight of component (c). A high incompatibility promotes thebleeding-out of component (c), i.e. component (c) bleeds out morequickly at lower temperatures and in larger quantities.

At the same time, the bleeding-out is promoted by a high mobility ofcomponent (c) in the polymerized matrix, i.e. by a high diffusioncoefficient. However, the diffusion coefficient of component (c)decreases with increasing molecular weight. Thus, on the one hand, ahigh molecular weight promotes the bleeding-out of component (c) throughan increase in the incompatibility, on the other hand it impairs thebleeding-out through a reduction in the diffusion coefficient. For agiven polymeric matrix, a range can be determined for the molecularweight, in which the bleeding-out rate is at a maximum. Through thevariation of the molecular weight of component (c), the bleeding-outbehaviour can therefore be controlled and adjusted.

The solubility behaviour depends, in addition, on the polarity of thepolymer matrix and of component (c). The properties of the polymermatrix are largely determined by component(s) (a). The polarity of thematrix can be increased, for example, by monomers with polar groups. Thebleeding-out is furthermore promoted by a high flexibility of thepolymer network. The flexibility of the polymer network can becontrolled by the degree of crosslinking and the type of the monomers.

An incompatibility suitable for the purposes according to the inventioncan be determined with reference to test bodies. The incompatibilitybrings about an increase in opacity during the polymerization. A highincompatibility manifests itself in a large increase in opacity, in thecase of a lower incompatibility, the increase in opacity turns out to becorrespondingly smaller. An advantageous incompatibility is present whenthere is only a low supersaturation during the polymerization, whichmanifests itself in that the test bodies are slightly opaque totransparent after the polymerization. If the test bodies are completelyopaque after the polymerization, this suggests a high incompatibility.

The components are matched to each other such that the desired thermalexpansion behaviour is achieved, i.e. that the maximum thermal expansionis below 1.5%, preferably below 1%, particularly preferably below 0.7%,wherein, according to a particularly preferred embodiment, the maximumexpansion is reached at a temperature of below 150° C., preferably below120° C., particularly preferably below 100° C. and quite particularlypreferably below 90° C.

The LCST lies below the temperature of the maximum thermal expansion,preferably 20 to 30° C. below this temperature. Materials with an LCSTbelow 120-130° C., preferably below 90-100° C., particularly preferablybelow 70-80° C. and quite particularly preferably below 60 to 70° C. arethus preferred according to the invention.

In addition, the LCST is preferably higher than the processingtemperature with the result that component (c) does not already bleedout during curing of the composition but only during subsequent warming.The LCST is preferably higher than 20° C., preferably higher than 30° C.and particularly preferably higher than 35° C.

It was found that the inert component (c) used according to theinvention and in particular the named preferred components arehomogeneously miscible with radically polymerizable monomers and inparticular with acrylates and methacrylates and do not impair thepolymerization of these monomers. During the stereolithographicprocessing of the compositions according to the invention in the liquidstate, a high reactivity as well as short exposure and processing timescan thereby be ensured.

The component (c) used according to the invention is characterized inaddition in that, during bleeding-out, it can be at least partiallyabsorbed by the investment material.

According to the invention those materials are preferred in whichcomponent (a) contains UDMA, TEGDMA or a mixture thereof. Particularlypreferred are materials in which component (a), in addition to UDMAand/or TEGDMA, contains at most 20 wt.-%, preferably at most 10 wt.-%,particularly preferably at most 5 wt.-% and quite particularlypreferably less than 3 wt.-% or no further monomers, wherein thepercentages relate to the total mass of component (a). Those materialsare preferred in particular which contain, as component (a), UDMA orTEGDMA, particularly preferably a mixture of UDMA, TEGDMA and SR339C orof UDMA, TEGDMA and bis-GMA and in particular a mixture of UDMA andTEGDMA. As component (c), in each case PPG with a molar weight of 1000to 4000 g/mol, preferably of 1500 to 3000 g/mol, quite particularlypreferably 2000 g/mol is preferred.

In the case of the monomers preferred according to the invention, PPGwith a molar weight of 2000 g/mol represents a good compromise betweenlow viscosity, sufficient phase incompatibility and sufficiently rapiddiffusion. Through the variation of the molecular weight of component(c) within the named ranges, the bleeding-out behaviour can be furtheroptimized for the desired application.

In the case of PEG-PPG-PEG, a molecular weight of approx. 3500 g/mol isadvantageous. The temperature of the maximum thermal expansion isshifted towards higher values through the use of this component.

In addition to the above-named components, the materials according tothe invention can advantageously contain further additives. Forstereolithographic use, materials are preferred which contain at leastone colorant and preferably also at least one polymerization inhibitor.

As colorant, organic dyes and pigments are preferred, in particular azodyes, carbonyl dyes, cyanine dyes, azomethines and methines,phthalocyanines and dioxazines. Dyes are particularly preferred whichare soluble in the materials, in particular azo dyes. Inorganic and inparticular organic pigments which can be dispersed well in the materialsare also suitable as colorant. Azo pigments and non-azo pigments arepreferred. Those substances are preferably used as colorant which burnout without residue. For this reason, organic pigments are preferred toinorganic pigments. In the case where inorganic pigments are used, thequantity thereof is preferably calculated such that the ash contentthereof after burn-out is below 0.1 wt.-%, relative to the total weightof the burn-out material.

The materials preferably contain at least one colorant which absorbs inthe same wavelength range as the polymerization initiator. Colorants areparticularly preferred which have an absorption maximum whichcorresponds to the wavelength of the light used for the curing.Colorants with an absorption maximum in the range of from 350 to 550 nm,preferably 380 to 480 nm, are particularly advantageous.

Colorants prevent the light used for the curing from penetrating toodeep into the materials and thus have an advantageous effect on theprecision of the components in the printing process, in particular inthe case of stereolithographic processes. Moreover, colorants can alsobe added for aesthetic purposes.

The polymerization inhibitor(s) serve(s) as stabilizer to prevent aspontaneous polyreaction. The inhibitors or stabilizers improve thestorage stability of the materials and also prevent an uncontrolledpolyreaction in the stereolithographic tank. The inhibitors arepreferably added in such a quantity that the materials arestorage-stable for a period of approx. 2-3 years. The inhibitors areparticularly preferably used in a quantity of 0.001 to 1.0 wt.-%, quiteparticularly preferably 0.001 to 0.20 wt.-%, in each case relative tothe total mass of the material.

So-called aerobic inhibitors are preferred, in particular phenols suchas hydroquinone monomethyl ether (MEHQ) or2,6-di-tert-butyl-4-methyl-phenol (BHT), which are really effective onlyin the presence of oxygen and are preferably used in a concentrationrange from 100-2000 ppmw. Suitable anaerobic inhibitors arephenothiazine, 2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO),iodine and copper(I) iodide. These act even in low concentrations ofpreferably 10-200 ppmw even in the absence of oxygen. A polymerizationdoes not take place until these additives have been consumed. It isadvantageous to use a mixture of aerobic and anaerobic inhibitors.

Aerobic inhibitors are preferably used in a quantity of 0.001 to 0.50wt.-% and anaerobic inhibitors in a quantity of 0.001 to 0.02 wt.-%, ineach case relative to the total mass of the material. Preferred mixturescontain 0.005-0.10 wt.-% aerobic inhibitors and 0.001 to 0.02 wt.-%anaerobic inhibitors, likewise relative to the total mass of thematerial.

The modelling materials according to the invention can also containparticulate filler. Generally, organic particles are preferred asfillers, in particular fillers which burn without forming ash. Thefiller(s) preferably has(have) a particle size of less than 25 μm,preferably less than 10 μm and particularly preferably less than 5 μm.All of the particle sizes herein, unless otherwise stated, are D50values, i.e. 50 vol.-% of the particles have a diameter which is smallerthan the stated value. Fillers serve primarily to adjust the viscosity,the mechanical and/or the optical properties of the materials.

The surface of the fillers can be modified, for example in order toimprove the dispersibility of the fillers in the organic matrix. Thosecompounds which are chemically bonded, i.e. by ionic or covalent bonds,to the surface of the fillers are preferably used for the surfacemodification. Compounds which contain either acid groups, preferablycarboxylic acid groups, phosphonic acid groups, hydrogen phosphategroups or acid phosphoric acid ester groups, or silyl groups, preferablyalkoxysilyl groups, are preferred. The particle surface can be partiallyor preferably completely covered with the modification agent. Themodification agents used according to the invention are monomericcompounds. Linear carboxylic acids, such as e.g. formic acid, aceticacid, propionic acid, octanoic acid, isobutyric acid, isovaleric acid,pivalic acid, or phosphonic acids, e.g. such as methyl-, ethyl-,propyl-, butyl-, hexyl-, octyl- or phenylphosphonic acid, areparticularly suitable as surface modification agents. As silylgroup-containing compounds, silanes such as propyltrimethoxysilane,phenyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane,trimethylchlorosilane, trimethylbromo-silane, trimethylmethoxysilane andhexamethyldisilazane are preferred. Quite particularly preferred surfacemodification agents are acid phosphoric acid esters such as e.g.dimethyl, diethyl, dipropyl, dibutyl, dipentyl, dihexyl, dioctyl ordi(2-ethylhexyl) phosphate. The surface modification agents can alsocomprise radically polymerizable groups, for example (meth)acrylategroups, which react with component (a) and are thus integrated into thepolymer network.

Preferred fillers are particulate waxes, in particular carnauba wax,preferably with a particle size of from 1 to 10 μm, crosslinkedpolymethyl methacrylate (PMMA) particles, preferably with a particlesize of from 500 nm to 10 μm, as well as polyamide-12 particles,preferably with a particle size of from 5 to 10 μm. Forstereolithography, fillers are preferably used the maximum particle sizeof which is smaller than the thickness of the stereolithographicallyproduced layers. Particles with a maximum size of 25 μm are preferred,preferably a maximum of 15 μm.

Filler-free materials are preferred according to the invention.

The rheological properties of the materials according to the inventionare adapted to the desired intended use. Materials forstereolithographic processing are preferably adjusted such that theirviscosity is in the range of from 50 mPa·s to 100 Pa·s, preferably 100mPa·s to 10 Pa·s, particularly preferably 100 mPa·s to 5 Pa·s. Theviscosity is determined at the desired processing temperature of thematerials with a cone and plate viscometer (shear rate 100/s). Theprocessing temperature is preferably in the range of from 10 to 70° C.,particularly preferably 20 to 30° C.

Materials for manual processing are preferably adjusted such that theyhave a paste-like consistency.

For the production of highly fluid materials, liquid monomers and highlyliquid components (c) are preferably used, whereas for the production ofmaterials with high viscosity, e.g. solid monomers and/or highly viscousor solid components (c) can also be used.

The materials according to the invention can also preferably contain inaddition one or more UV absorbers. Preferred UV absorbers are2,2′-methylenebis[6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol](CAS No. 103597-45-1), 2,2′,4,4′-tetrahydroxybenzophenone (CAS No.131-55-5), 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol(CAS No. 3896-11-5), 2,2′-benzene-1,4-diylbis(4H-3,1-benzoxazin-4-one)(CAS No. 18600-59-4),2-(4,6-bis-(2,4-dimethylphenyl)-1,3,5-triazin-2-yl)-5-(octyloxy)phenol(CAS No. 2725-22-6), 2-(2-hydroxy-5-methylphenyl)benzotriazole (CAS No.2440-22-4) and 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-methylphenol (CASNo. 23328-53-2).

Also suitable are so-called Hindered Amine Light Stabilizers such asbis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate (CAS No. 41556-26-7) andmethyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate (CAS No. 82919-37-7),bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate (CAS No.129757-67-1), bis(1,2,2,6,6-pentamethyl-4-piperidyl)[[3,5-bis(1,1-di-methyl-ethyl)-4-hydroxyphenyl]methyl]butylmalonate (CASNo. 63843-89-0).

In addition to the above-named components, the materials according tothe invention can contain one or more further additives, which arepreferably selected from thickeners, optical brighteners (e.g. LumiluxLZ Blue, CAS No. 658084-50-5, or2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene, CAS No. 7128-64-5) and,in the case of materials containing fillers, from dispersants.

The materials according to the invention preferably have the followingcomposition:

-   -   20 to 90 wt.-%, preferably 42.5 to 82.5 wt.-%, particularly        preferably 50 to 79.3 wt.-% component (a),    -   0.1 to 5 wt.-%, preferably 0.3 to 2.5 wt.-%, particularly        preferably 0.5 to 1.5 wt.-% (photo)initiator (b) and    -   9.9 to 79.9 wt.-%, preferably 15 to 55 wt.-%, particularly        preferably 20 to 48.5 wt.-% inert component (c).

A quite particularly preferred composition contains:

-   -   54.7 to 77.7 wt.-%, preferably 64 to 74 wt.-% component (a),        preferably UDMA, TEGDMA, a mixture of UDMA and TEGDMA or a        mixture of UDMA, TEGDMA and bis-GMA,    -   0.5 to 1.5 wt.-%, preferably 0.6 to 1.2 wt.-%        (photo)initiator (b) and    -   23.8 to 44.8 wt.-%, preferably 24.8 to 35.4 wt.-% inert        component (c), preferably PPG with a molecular weight of 1000 to        4000 g/mol, particularly preferably 1500 to 3000 g/mol,        co-PEG-PPG with a molecular weight of 1000 to 10,000 g/mol,        particularly preferably 1500 to 5000 g/mol, quite particularly        preferably 2000 to 4000 g/mol and in particular polypropylene        glycol with a molecular weight of approx. 2000 g/mol.

Most preferred is a composition, which contains

-   -   64 to 74 wt.-% UDMA, TEGDMA, a mixture of UDMA and TEGDMA or a        mixture of UDMA, TEGDMA and bis-GMA as component (a),    -   0.6 to 1.2 wt.-% photoinitiator (b) and    -   24.8 to 35.4 wt.-% polypropylene glycol (PPG) with a molecular        weight of 1000 to 4000 g/mol, particularly preferably 1500 to        3000 g/mol and in particular with a molecular weight of approx.        2000 g/mol as inert component (c).

Furthermore, the materials according to the invention preferablyadditionally contain:

-   -   0.0001 to 1 wt.-%, preferably 0.0001 to 0.5 wt.-%, particularly        preferably 0.0001 to 0.2 wt.-% colorant; and/or    -   0.0001 to 2 wt.-%, preferably 0.0001 to 1 wt.-%, particularly        preferably 0.0001 to 0.5 wt.-% UV absorber; and/or    -   0 to 40 wt.-%, preferably 0 to 30 wt.-%, particularly preferably        1 to 20 wt.-% organic filler.

Moreover, the materials preferably contain

-   -   0 to 5 wt.-%, preferably 0 to 3 wt.-%, particularly preferably 0        to 2 wt.-% further additive(s).

Unless otherwise stated, all data relate to the total weight of thematerial. The individual constituents are preferably selected from theabove-named preferred components.

The materials according to the invention are suitable in particular forthe production of models for castings made of metal or glass ceramic, inparticular for the production of models for dental restoration, such ase.g. inlays, onlays, veneers, crowns, bridges or frameworks as well asremovable (partial) prostheses.

A subject of the invention is furthermore a process for the productionof dental restorations in which,

-   (A) with a modelling material according to the invention, a model of    the tooth to be restored or of the teeth to be restored is moulded,-   (B) the model is then invested in an investment material,-   (C) after the investment material has set, the invested model is    heated in a furnace until the modelling material is completely    removed from the mould,-   (D) an alloy or a glass ceramic material is poured or pressed into    the mould produced in this way.

The production of the model in step (A) preferably takes place by astereolithographic process. For this purpose, a virtual image of thetooth position is generated on the computer by the direct or indirectdigitization of the tooth to be restored or of the teeth to be restored,a model of the dental restoration is then designed on the computer onthe basis of this image and this is then produced by additivestereolithographic manufacturing.

In step (B), the model can then be invested in an investment material ina conventional manner. The materials according to the invention aresuitable for use with conventional dental investment materials. Adistinction is drawn between investment materials containing plaster andthose without. Plaster-containing investment materials contain plasterand quartz as main constituents in the ratio 1:2 to 1:4. Plaster servesas a binder. A preferred modification of quartz is cristobalite.Plaster-free investment materials which contain phosphates as binder arepreferred. Phosphate-bonded investment materials contain SiO₂modifications which are resistant to high temperatures, and a binder. Amixture of magnesium oxide and ammonium phosphate (NH₄H₂PO₄) ispreferably used as binder. The mixtures set after mixing with water.

The removal of the model from the mould in step (C) takes place byheating e.g. in a furnace to a temperature of preferably 600° C. to1000° C. The heating can take place in one or more stages. It ispossible but not absolutely necessary to raise the temperature to thedesired end temperature in a controlled manner. During the heating,component (c) bleeds out totally or partially, flows out of the mouldand/or is absorbed by the investment material. The mould is then heatedfurther, wherein residual model material is thermally decomposed.

In step (D), a metal alloy is poured or a suitable glass ceramicmaterial is pressed into the negative mould of the original model thusproduced. Metal alloys suitable for dental purposes or glass ceramicmaterials suitable for dental purposes are preferably used.

BRIEF DESCRIPTION OF THE FIGURES

The invention is explained in more detail below by means of figures andembodiment examples.

FIG. 1 shows the thermal expansion curve of the material according tothe invention from Example 2, measured on cylinders with a 6 mm diameterand a height of 6 mm. The measurement was carried out using a Q400measuring device from TA Instruments with a macro-expansion probe.

FIG. 2 shows the thermal expansion curve of the material according tothe invention from Example 4.

FIG. 3 shows the thermal expansion curve of the material according tothe invention from Example 5.

FIG. 4 shows the thermal expansion curve of the material according tothe invention from Example 6.

FIGS. 5 and 6 show the expansion curves of materials customary in thetrade according to the state of the art.

FIGS. 7 and 8 show the thermal expansion curves of comparison materialswhich do not contain any inert component (c).

FIGS. 9 to 11 show dental bridge frameworks made of glass ceramic whichwere produced using the materials according to the invention fromExamples 4 (FIG. 9), 5 (FIG. 10) and 8 (FIG. 11), directly after thedivesting. The bridge frameworks do not exhibit any pressing defects.

FIGS. 12 to 14 show dental bridge frameworks made of glass ceramic whichwere produced using comparison materials without component (c) (FIG. 12,material V1; FIG. 13, material V2; FIG. 14, material V3). The bridgeframeworks exhibit clearly visible pressing defects.

FIGS. 15 and 16 show dental bridge frameworks made of glass ceramicwhich were produced using materials customary in the trade. The bridgeframeworks exhibit clearly visible pressing defects.

EXAMPLES Examples 1 to 9

Modelling Materials

The components listed in Table 1 were mixed with each otherhomogeneously in the stated quantities. The components were weighed outand stirred at approx. 50° C. for 1 hour and then at room temperaturefor approx. 16 h (overnight). In the case of pigment- andfiller-containing compositions, the pigment and the filler respectivelywere stirred into UDMA, the substances were then homogenized anddispersed three times with a three-roll mill with a gap width of 10 μmand then stirred into the remaining, already dissolved organic matrix(at least 1 hour at room temperature). Finally, optional furtheradditives such as thickeners were added to the paste and stirred againfor at least 1 hour.

TABLE 1 Modelling materials for stereolithography Composition [wt.-%]Component 1 2 3 4 5 6 7 8 9 Monomer (a) UDMA¹⁾ 42 44 49 48.95  49 36.1534 49 32 TEGDMA²⁾ 27 25 10.04 20    20 20.88 17 20 27 Bis-GMA³⁾ — — — —— — — — 10 Initiator (b) Irgacure 819⁴⁾ — — — 0.95 — — — — TPO⁵⁾ — 0.950.95 — 0.95 0.92    0.95 0.95 0.95 Inert component (c) PPG 2000⁶⁾ 30 3040 30    30 30 — 20 30 PPG 1500⁷⁾ — — — — — — 20 — — PPG 4000⁸⁾ — — — —— —  8 — — PPG 400⁹⁾ — — — — — — — 10 — Dye Sudan IV¹⁰⁾    0.05 — 0.010.05 0.05 0.05    0.05 0.05 0.05 Sudan Black B¹¹⁾ — 0.05 — — — — — — —White pigment (TiO₂)¹²⁾ — — — 0.05 — — — — — Further components Waxparticles¹³⁾ — — — — — 10 — — — Thickener¹⁴⁾ — — — — — 2 — — — PMMAparticles¹⁵⁾ — — — — — — 20 — — ¹⁾Urethane dimethacrylates (CAS No.72869-86-4) ²⁾Triethylene glycol dimethacrylate (CAS No. 109-16-0)³⁾Addition product of methacrylic acid and bisphenol A diglycidyl ether⁴⁾Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (CAS No. 162881-26-7)⁵⁾Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (CAS No. 75980-60-8)⁶⁾Polypropylene glycol) (CAS No. 25322-69-4), MW = 2000 g/mol⁷⁾Polypropylene glycol) (CAS No. 25322-69-4), MW = 1500 g/mol⁸⁾Polypropylene glycol) (CAS No. 25322-69-4), MW = 4000 g/mol⁹⁾Polypropylene glycol) (CAS No. 25322-69-4), MW = 400 g/mol ¹⁰⁾CAS No.85-83-6 ¹¹⁾CAS No. 4197-25-5 ¹²⁾Titanium dioxide white pigment, particlesize D50 <500 nm ¹³⁾MC6015, micronized carnauba wax, d = 1-10 μm¹⁴⁾Solution of a high-molecular-weight, urea-modified, medium polarpolyamide (Byk 430) ¹⁵⁾highly crosslinked PMMA, Chemisnow MX80H3wT(Soken Chemical & Engineering Co., Ltd., Japan), D50 = 800 nm

Example 10 Modelling Materials—Comparison Examples

Analogously to Examples 1 to 9, the comparison materials listed in Table2 were prepared.

TABLE 2 Comparison materials for stereolithography Composition [wt.-%]Component V1* V2* V3* Monomer (a) UDMA¹⁾ — 74   40 TEGDMA²⁾ — 25   25SR348C¹⁶⁾ 59 — — SR480¹⁷⁾ 40 — — Initiator (b) TPO⁵⁾ 0.95 0.9 0.95 Inertcomponent (c) — — — Dye Sudan IV¹⁰⁾ 0.05 0.1 0.05 Further components Waxparticles¹³⁾ — — 34 *Comparison material ¹⁻¹⁵⁾see Table 1 ¹⁶⁾Bisphenol Adimethacrylate with 3 ethoxy groups (Sartomer) ¹⁷⁾Bisphenol Adimethacrylate; ethoxylated 10 times on average

Example 11

Measurement of the Thermal Expansion

For the determination of the thermal expansion of the materials,cylinders with a 6 mm diameter and a height of 6 mm were producedstereolithographically with a printer from the materials described inTables 1 and 2. The cylinders were cured in layers and then post-exposedin a post-exposure device at a wavelength of 400 nm with an intensity of10 mW/cm² for 5 minutes. The cylinders were then placed in the samplechamber of a thermomechanical analyzer (Q400 type from TA Instrumentswith a macro-expansion probe) and heated at a heating rate of 5 K/min to800° C. The linear thermal expansion of the cylinders during heating upand the thermal decomposition in the temperature range of from 30° C. to800° C. was measured in an air atmosphere. The contact force of themeasuring probe was 0.1 N.

In FIGS. 1 to 4, the expansion curves for the materials from Examples 2,4, 5 and 6 are shown. In all cases, the thermal expansion remainedsignificantly below 0.7% over the whole measurement range. The testsprove that the thermal expansion of the materials is compensated for themost part by the bleeding-out of component (c). The maximum linearthermal expansion was reached in all cases at a temperature below 90° C.Thereafter, the length of the cylinders remained below the maximumvalue. From 300° C., the materials decomposed thermally and burned outwithout residue (residue on ignition in all cases <0.1 wt.-%). Thedecomposition manifests itself in a steep fall in the linear thermalexpansion.

Example 6 shows that the wax particles used as filler do not impede thebleeding-out of component (c) and have practically no influence on theexpansion.

The expansion curves of the other examples are similar. In Example 8,the maximum thermal expansion of <1.5% is reached at 125° C.

For comparison, the expansion curves of conventional, commerciallyavailable stereolithography materials for the lost-wax technique weredetermined. FIG. 5 shows the expansion curve of the Bego materialVarseoWax CAD/Cast. The material expands by more than 4%, and thetemperature of the maximum expansion is only reached shortly before thethermal decomposition, i.e. at more than 300° C.

In FIG. 6, the expansion curve is shown of a further stereolithographyresin for the lost-wax technique customary in the trade (3D Systems'Visijet FTX Cast). The material expands by more than 2%, and thetemperature of the maximum thermal expansion is above 200° C.

FIGS. 7 and 8 show the expansion curves of comparison materials V1 andV3. Material V1 expands by more than 6% (FIG. 7); material V3 by morethan 8% (FIG. 8). In both cases, the temperature of the maximumexpansion was only reached shortly before the thermal decomposition,i.e. at more than 300° C. or more than 200° C., respectively.

The measurements show that the maximum thermal expansion of thecomparison materials is greater in all cases than that of the materialsaccording to the invention. By adding component (c), the thermalexpansion can be effectively reduced. Moreover, in the case of thecomparison materials, the maximum linear thermal expansion is atsignificantly higher temperatures, which are determined by thedecomposition temperature of the materials.

Example 12

Production of Models

With a 3D printer, models of a three-unit bridge were manufactured fromthe materials described in Examples 1 to 9 and with materials customaryin the trade. In all cases, the same dataset was used to produce themodels.

The models were built up on a ring base with the compatible ring gaugeand provided with pressing channels. The models were then invested ineach case in a phosphate-based investment material customary in thetrade (200 g PressVest Speed; Ivoclar Vivadent AG). The fine investmentof the cavities was undertaken with a small brush. The invested ring wasallowed to set without vibration for 35 minutes. The rings were thenplaced directly in the preheating furnace preheated to 850° C. and leftthere at 850° C. for 1.5 h, in order to remove the models from themcompletely. The rings were then taken out of the preheating furnace andthe hot rings were fitted with a ceramic ingot (IPS e.max Press ingot,Ivoclar Vivadent AG), placed in the hot press furnace (Programat EP5010, Ivoclar Vivadent) and the chosen press program was started.

After the end of the pressing procedure, the rings were taken out of thefurnace and placed on a cooling grid in a place protected from draughtsfor cooling. After cooling to room temperature, the rings were separatedusing a separating disc and the pressed objects were divested. The roughdivesting took place using polishing jet medium at 4 bar pressure; thefine divesting using polishing jet medium at 2 bar pressure. The pressresults were assessed directly after the fine divesting.

FIGS. 9 to 11 show bridge frameworks by way of example, which wereproduced using the materials according to the invention from Examples 4,5 and 8. The pictures were produced directly after the divesting. Nopressing defects are visible. The bridges produced with the othermaterials according to the invention were similar, in no case werepressing defects present.

In contrast thereto, the bridges manufactured using the comparisonmaterials exhibited clearly visible pressing defects, which can beattributed to cracks in the mould which formed during the expansion ofthe modelling materials.

FIGS. 12 to 14 show the bridge frameworks obtained using the comparisonmaterials V1 to V3. FIG. 15 shows a bridge manufactured using the Begomaterial Varseo Wax CAD/Cast customary in the trade and FIG. 16 shows abridge manufactured using the 3D Systems product Visijet FTX Cast. Inall cases, pressing defects are present, which make a more or lesscomplex finishing of the restorations necessary. Some of therestorations were unusable.

Even the bridge obtained using the 3D Systems product Visijet FTX Castexhibited pressing defects, although, at approx. 2%, this material has arelatively small maximum thermal expansion. In contrast thereto, thebridge obtained with the material according to the invention fromExample 8 (FIG. 11; max. expansion <1.5%) was fault-free. In the case ofthe material according to the invention, the maximum thermal expansionis reached at 125° C.; in the case of the comparison material only atabove 200° C.

The comparison material V3 contained wax particles but no component (c).The wax particles melted on burn-out, but the wax could only flow out ofthe surface of the component and remained trapped in the centre. Themodel expanded greatly on burn-out; the press results werecorrespondingly poor.

The invention claimed is:
 1. Process for the production of dentalrestorations in which, (A) a model of a tooth to be restored or of teethto be restored is moulded, wherein the model is fabricated of acomposition comprising a radically polymerizable composition whichcomprises (a) at least one radically polymerizable monomer, (b) at leastone initiator for the radical polymerization and (c) at least one inertcomponent, characterized in that the inert component (c) is soluble inthe polymer formed by polymerization of the monomer(s) (a), wherein thesolubility of component (c) decreases as the temperature increases, withthe result that a phase separation takes place above a particulartemperature, (B) the model is then invested in an investment material,(C) after the investment material has set, the invested model is heatedin a furnace so that the modelling material is completely removed fromthe mould, (D) an alloy or a ceramic or glass ceramic material is pouredor pressed into the mould.
 2. Process according to claim 1, in which theradically polymerizable composition comprises as component (a), at leastone (meth)acrylate and/or (meth)acrylamide.
 3. Process according toclaim 2, wherein the at least one (meth)acrylate and/or (meth)acrylamidecomprises one or more mono- or multifunctional (meth)acrylates or amixture thereof.
 4. Process according to claim 1, which comprises, asinitiator (b), a photoinitiator.
 5. Process according to claim 4, whichcomprises, as the photoinitiator, benzophenone, benzoin or a derivativethereof, an *α-diketone or a derivative thereof,9,10-phenanthrenequinone, 1-phenyl-propane-1,2-dione, diacetyl,4,4′-dichlorobenzil, camphorquinone (CQ),2,2-dimethoxy-2-phenyl-acetophenone, an *α-diketone in combination withan amine as reducing agent, a Norrish type I photoinitiator, a monoacyl-or bisacylphosphine oxide, a monoacyltrialkyl- or diacyldialkylgermaniumcompound, benzoyltrimethylgermanium, dibenzoyldiethylgermanium,bis(4-methoxybenzoyl)diethylgermanium (MBDEGe), a mixture ofbis(4-methoxybenzoyl)diethylgermanium in combination with camphorquinoneand 4-dimethylaminobenzoic acid ethyl ester, camphorquinone (CAS No.10373-78-1) in combination with ethyl 4-(dimethylamino)benzoate (EMBO,CAS No. 10287-53-3), 2,4,6-trimethylbenzoyl diphenylphosphine oxide(TPO, CAS No. 75980-60-8), ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate (TPO-L, CAS No. 84434-11-7),phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819, CAS162881-26-7), bis(2,6-difluoro-3-(1-hydropyrrol-1-yl)phenyl)titanocene(Irgacure 784, CAS No. 125051-32-3),2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone (Irgacure 369, CASNo. 119313-12-1), 1-butanone,2-(dimethylamino)-2-(4-methylphenyl)methyl-1-4-(4-morpholinyl)phenyl(Irgacure 379, CAS No. 119344-86-4) and/orbis(4-methoxybenzoyl)diethylgermanium (MBDEGe; K69).
 6. Processaccording to claim 1, which comprises, as inert component (c), at leastone phthalate, polyethylene glycol (PEG), polypropylene glycol (PPG),PEG-PPG copolymer, glycerol derivative, ethoxylated and/or propoxylatedglycerol, ethylenediamine tetrakispropoxylate, ethylenediaminetetrakisethoxylate or a mixture thereof.
 7. Process according to claim6, in which the radically polymerizable composition comprises, ascomponent (c) at least one of PPG with a molecular weight of 1000 to4000 g/mol, PPG with a molecular weight of 1500 to 3000 g/mol,co-PEG-PPG with a molecular weight of 1000 to 10,000 g/mol, co-PEG-PPGwith a molecular weight of 1500 to 5000 g/mol and/or polypropyleneglycol with a molecular weight of approx. 2000 g/mol.
 8. Processaccording to one of claim 1, in which the radically polymerizablecomposition additionally comprises at least one colorant and/or oneinhibitor.
 9. Process according to claim 1, in which the radicallypolymerizable composition comprises 20 to 90 wt.-% component (a), 0.1 to5 wt.-% (photo)initiator (b) and 9.9 to 79.9 wt.-% inert component (c),in each case relative to the total mass of the composition.
 10. Processaccording to claim 1, in which the radically polymerizable compositioncomprises 42.5 to 82.5 wt.-% component (a), 0.3 to 2.5 wt.-%(photo)initiator (b) and 15 to 55 wt.-% inert component (c), in eachcase relative to the total mass of the composition.
 11. Processaccording to claim 1, in which the radically polymerizable compositioncomprises 50 to 79.3 wt.-% component (a), 0.5 to 1.5 wt.-%(photo)initiator (b) and 20 to 48.5 wt.-% inert component (c), in eachcase relative to the total mass of the composition.
 12. Processaccording to claim 8, in which the radically polymerizable compositioncomprises 54.7 to 77.7 wt.-% component (a) comprising UDMA, TEGDMA, amixture of UDMA and TEGDMA or a mixture of UDMA, TEGDMA and bis-GMA, 0.5to 1.5 wt.-% (photo)initiator (b) and 23.8 to 44.8 wt.-% inert component(c) comprising one or more of PPG with a molecular weight of 1000 to4000 g/mol, co-PEG-PPG with a molecular weight of 1000 to 10,000 g/mol,and polypropylene glycol with a molecular weight of approx. 2000 g/mol,in each case relative to the total mass of the composition.
 13. Processaccording to claim 8, in which the radically polymerizable compositioncomprises 64 to 74 wt.-% component (a) comprising UDMA, TEGDMA, amixture of UDMA and TEGDMA or a mixture of UDMA, TEGDMA and bis-GMA, 0.6to 1.2 wt.-% (photo)initiator (b) and 24.8 to 35.4 wt.-% inert component(c) comprising one or more of PPG with a molecular weight of 1500 to3000 g/mol, co-PEG-PPG with a molecular weight of 1500 to 5000 g/mol,co-PEG-PPG with a molecular weight of 2000 to 4000 g/mol andpolypropylene glycol with a molecular weight of approx. 2000 g/mol, ineach case relative to the total mass of the composition.
 14. Processaccording to claim 9, in which the radically polymerizable compositionadditionally comprises 0.0001 to 1 wt.-% colorant; and/or 0.0001 to 2wt.-% UV absorber; and/or 0 to 40 wt.-% organic filler; and/or 0 to 5wt.-% further additive(s), in each case relative to the total mass ofthe composition.
 15. Process according to claim 9, in which theradically polymerizable composition additionally comprises 0.0001 to 0.5wt.-% colorant; and/or 0.0001 to 1 wt.-% UV absorber; and/or 0 to 30wt.-% organic filler; and/or 0 to 3 wt.-% further additive(s), in eachcase relative to the total mass of the composition.
 16. Processaccording to claim 9, in which the radically polymerizable compositionadditionally comprises 0.0001 to 0.2 wt.-% colorant; and/or 0.0001 to0.5 wt.-% UV absorber; and/or 1 to 20 wt.-% organic filler; and/or 0 to2 wt.-% further additive(s), in each case relative to the total mass ofthe composition.
 17. Process according to claim 1, in which theradically polymerizable composition has a maximum linear thermalexpansion below 1.5%.
 18. Process according to claim 1, in which theradically polymerizable composition has a maximum linear thermalexpansion below 0.7%.
 19. Process according to claim 17, in which themaximum thermal expansion is reached at a temperature of below 150° C.20. Process according to claim 17, in which the maximum thermalexpansion is reached at a temperature of below 90° C.
 21. Processaccording to claim 1, in which the model is produced in step (A) by astereolithographic process.
 22. Process according to claim 1, in which,(A) the model of the tooth to be restored or of the teeth to be restoredis fabricated of a composition comprising a radically polymerizablecomposition which comprises (a) 20 to 90 wt.-% of at least one radicallypolymerizable monomer comprising at least one (meth)acrylate and/or(meth)acrylamide (b) 0.1 to 5 wt.-% of at least one initiator for theradical polymerization and (c) 9.9 to 79.9 wt.-% of at least one inertcomponent, in each case relative to the total mass of the composition,wherein said inert component (c) is soluble in the polymer formed bypolymerization of the monomer(s) (a), wherein the solubility ofcomponent (c) decreases as the temperature increases, with the resultthat a phase separation takes place above a particular temperature, andwherein component (c) comprises at least one of PPG with a molecularweight of 1000 to 4000 g/mol and co-PEG-PPG with a molecular weight of1000 to 10,000 g/mol, (B) the model is then invested in an investmentmaterial, (C) after the investment material has set, the invested modelis heated in a furnace so that the modelling material is completelyremoved from the mould, (D) an alloy or a ceramic or glass ceramicmaterial is poured or pressed into the mould.
 23. Process according toclaim 22, in which the radically polymerizable composition of step (A)comprises (a) 50 to 79.3 wt.-% of at least one radically polymerizablemonomer, comprising at least one (meth)acrylate and/or (meth)acrylamide,(b) 0.5 to 1.5 wt.-% of at least one initiator for the radicalpolymerization and (c) 20 to 48.5 wt.-% of at least one inert component,in each case relative to the total mass of the composition.